Note on one French rural deity (4)
I believe the answer is:
faun
'rural deity' is the definition.
(woodland spirit in classical mythology)
'note on one french' is the wordplay.
'note' becomes 'fa' (fourth note in sol-fa notation: doh, ray, me...).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'one french' becomes 'un' ('one' in French).
'fa'+'un'='FAUN'
(Other definitions for faun that I've seen before include "Roman goat-like deity" , "Rural deity, half-man, half-goat" , "Deity in human shape, with horns and a goat's tail" , "This old deity" , "Rural deity with the form of a man-goat" .)